How genes specify a complex multicellular structure, such as neuromuscular network, is completely unknown: e.g., how many genes define each cell and each intercellular contact? How many cells are affected by each gene, and what features do such cells have in common? How are the temporal and spatial aspects of the developmental program encoded in the genome? I propose to explore these problems by genetically analyzing the anatomically simple egg-laying system of the small soil nematode Caenorhabditis elegans. C. elegans is eminently suited for genetics, and much is known about the structure and development of its egg-laying system. Mutants defective in egg-laying will be isolated and then characterized anatomically, developmentally, behaviorally, and, in a few cases, biochemically. Genetics will provide a tool with which to define and purify those mutant strains to be examined in detail. Genetics will also be used analytically, to reveal the roles of and interrelationships among the different genes which specify the egg-laying system. This system will be further characterized by pharmacological and behavioral studies of wild-type C. elegans, as well as by comparative studies with related nematode species which do not lay eggs. These experiments should define the genes which control the structure, function, and development of the egg-laying system of C. elegans and may provide some general insights concerning the genetic specification of a behavioral network.